`skill_manage(action='patch')` rejected a missing `old_string` with:
old_string is required for 'patch'. Provide the text to find.
That is a dead end. The model cannot tell whether it omitted the argument or
supplied text that did not match, so it retries blindly — and then escapes to
the neighbour that always works: `action='write_file'`, which rewrites the
entire skill file and destroys unrelated content. `skill_manage`'s own action
enum puts that destructive path one token away from the failing one.
The error now names the recovery route: `old_string` must be the EXACT text
currently in the file, read the target first (the skill's SKILL.md, or the
file named by `file_path`), copy the snippet verbatim, and do not fall back
to `action='write_file'`.
Validation lives in `_patch_skill` rather than the dispatcher. `skill_manage()`
previously returned its own bare missing-argument error before ever calling
the helper, which would leave the new guidance unreachable through the public
tool. Removing that duplicate makes the helper the single source of truth; its
{"success": False, "error": ...} flows through the same json.dumps path, so
the serialized shape is unchanged, and validation still precedes the skill
lookup — a missing old_string on an unknown skill still reports the argument
error rather than "skill not found".
Fixes#33064
Clifford Garwood
4f4e778db8